In the crushing darkness of the hadal zone—deep ocean trenches plunging 6,000 m to nearly 11,000 m—scientists have uncovered a hidden community. A study published in Science on May 14 reports the discovery of a protist-dominated hard-substrate fauna across seven hadal regions in Oceania, highlighting an overlooked yet highly active carbon "hotspot."

The research was led by the Institute of Deep-Sea Science and Engineering (IDSSE) of the Chinese Academy of Sciences (CAS) under the Global Hadal Exploration Program (GHEP).

The hadal zone contains extensive hard substrates along trench walls, fracture zones, and subducted seamounts. Since recovering rock samples from these extreme environments is challenging, the fauna on these surfaces remained largely unknown until now.

The fauna reported in this study comprises 32 species across six phyla—most new to science—including a new monothalamous foraminiferal family (Plumettidae) and a new bryozoan family (Pierrellidae).

The organisms are millimeter-sized and reach densities up to 4,300 individuals per dm². The dominant filamentous life forms were identified as agglutinated foraminifera—known as "rock feathers" or shirong ("石茸") in Chinese. For years, the small size, simple morphology, and abundant sediment of these species complicated DNA extraction, thereby obscuring their biological affinity.

Dominant species of the deepest hard-substrate fauna in the Kermadec and Mariana trenches. (Image by IDSSE)

The researchers further investigated how these organisms obtain energy. In this study, no typical chemosynthetic indicator taxa, such as siboglinid tubeworms or vesicomyid clams, were observed across all the hard-substrate dives. Metagenomic analysis also failed to detect any known chemolithoautotrophic symbionts in the dominant species. Instead, terrestrial pine pollen grains, many in different stages of digestion, confirmed a heterotrophic diet. This finding challenges the previous chemolithoautotrophic hypothesis.

Using in situ seafloor imagery and samples from 98 dives of the manned submersible Fendouzhe, the researchers documented the widespread distribution of this fauna, especially at depths of 9,000–10,898 m in the Kermadec and Mariana trenches. Other recent GHEP cruises revealed similar assemblages in five additional trenches (Aleutian, Kuril–Kamchatka, Puysegur, Atacama, and Mussau), suggesting that such communities may be widely distributed in hadal trenches worldwide.

The sessile foraminifera contribute an estimated 2–11% of total eukaryotic biomass carbon in global hadal trenches, highlighting a previously overlooked but highly active carbon "hotspot," with implications for deep-sea carbon cycling and reassessment of hadal biological pump efficiency.

The study also extends maximum depth records for several marine taxa: the deepest known bryozoan (Pierrella fendouzhei sp. nov., 9,981 m), scyphozoan polyps (9,982 m), and hydrozoan polyps (9,195 m). The deepest bryozoan belongs to an ancient lineage first described from Cretaceous shallow-water strata, suggesting that the hadal zone may serve as a refuge for archaic taxa.

The V-shaped trench topography funnels organic matter downslope, and turbidity currents frequently scour rock surfaces. Agglutinated foraminifera primarily colonize vertical lateral surfaces of rock outcrops, where the impact of turbidity currents is reduced. They hang downward from attachment points, a posture interpreted as a strategy to avoid burial by sediment while efficiently capturing upslope-moving organic particles. The biodiversity, biogeography, functional ecology, and global carbon-cycle roles of these deep hard-substrate faunas remain open questions.

The first and corresponding authors of the study are, respectively, Dr. SONG Xikun and Dr. PENG Xiaotong from IDSSE. Collaborating institutions include Earth Science New Zealand, the National Oceanography Centre (UK), the Natural History Museum (UK), the Bavarian State Collection of Zoology (Germany), the University of Vienna (Austria), the National Research and Innovation Agency (Indonesia), Tsinghua University, Xiamen University, and the Institute of Software of CAS.

Initiated by IDSSE, GHEP is a ten-year United Nations Ocean Decade program dedicated to exploring the deepest parts of the global ocean. It employs full-ocean-depth submersibles, including Fendouzhe, to conduct research on hadal geology, biology, and environments.

The study was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the International Partnership Program of CAS, the Hainan Province Science and Technology Special Fund, the IDSSE project, and the Global Trench Exploration and Diving Program.

Maps of 98 manned submersible dive sites in seven hadal regions around Oceania. (Image by IDSSE)